The present invention relates to an ink-jet image forming method and an ink-jet image forming device which use a fast-drying ink and a slow-drying ink together, and in particular to an ink-jet image forming method and an ink-jet image forming device for improving image quality by increasing reproducibility of color of the slow-drying ink while maintaining a constant drying time for the slow-drying ink, e.g., a black ink. The invention also relates to improvement in preventing contamination due to contact between recording sheets after image formation when images successively formed with respect to recording sheets which are continuously fed.
In image forming devices employing an ink-jet system (referred to as xe2x80x9cink-jet printerxe2x80x9d hereinafter where appropriate), improvement in dot forming method is sought for to improve image quality and reduce drying time.
For example, U.S. Pat. No. 5,596,355 (publication date: Jan. 21, 1997) discloses a technique of forming an image with the use of a slow-drying black ink, having high black reproducibility but slow drying time, and a fast-drying black ink, which dries fast but its print density is low. In this technique, when color dots are formed adjacent to the area where black dots are formed, the fast-drying black ink is used, or inks of C, M, Y are overlaid to make up the boundary area, and the slow-drying black ink is used to form the other area.
This improves reproducibility of black and suppresses mixing of black dots and color dots at the boundary.
Further, Japanese Unexamined Patent Publication No. 149036/1995 (Tokukaihei 7-149036) (publication date: Jun. 13, 1995) discloses a technique which uses a black ink which has low permeability with respect to a recording sheet, and inks of C, M, and Y which have high permeability. FIG. 37 and FIG. 38 show an example of how dots are formed by this technique. That is, when a color dot area is formed adjacent to a black dot area, the black dots in the black dot area are interpolated and color dots are formed instead therein (staggered dots are formed). Further, it also teaches forming color dots as an underlying layer of a black dot area so that the black dots are formed over on the color dots.
This is intended to prevent mixing of the black dot area and the color dot area and to reduce a drying time of the black dots.
Further, Japanese Unexamined Patent Publication No. 197831/1996 (Tokukaihei 8-197831) (publication date: Aug. 6, 1996) discloses a technique similar to that of the foregoing publication No. 7-149036.
Further, Japanese Unexamined Patent Publication No. 338136/1993 (Tokukaihei 5-338136) (publication date: Dec. 21, 1993) discloses finding a proportion of black dots in an image to be formed and an ambient temperature of image formation, and changing a transport speed, etc., of a recording sheet with an ink based on the proportion of the black dots and the ambient temperature thus found, so as to ensure drying of the ink on the recording sheet.
Also, apart from the improvement in print method as above, there have been many proposals for reducing drying time of prints by the provision of drying means which performs heating using a halogen lamp, for example.
However, in the technique disclosed in the foregoing U.S. Pat. No. 5,596,355, the following problems are caused in forming a high black dot density area (referred to as xe2x80x9csolid black areaxe2x80x9d hereinafter).
That is, in this technique, because the solid black area is formed with the slow-drying black ink having high reproducibility of black to improve image quality, there are cases where the prints are contaminated or black is seen through the back of the sheet due to the black ink which has not been dried sufficiently. This is due to a correlation between black dot density and drying time, whereby the drying time becomes longer in a solid black area where the black dot density is high, as in characters of 10 points or larger, or lines of 0.5 point or larger, and which exceeds a certain area.
In particular, in ink-jet printers which employ the face-down system to improve operability, the problem of sticking ink to the transport roller, which first comes into contact with the print surface immediately after the print process, and the problem of re-transfer of an ink to the recording sheet become more pronounced.
On the other hand, when the fast-drying black ink is used to form the solid black area, image quality suffers because reproducibility of black is poor.
Further, the foregoing publication No. 7-149036 and No. 8-197831 have the problem of poor image quality of black due to color dots, i.e., due to co-existing monochromatic color of yellow (Y), magenta (M), or cyan (C) in a boundary area in the black dot area.
Further, the foregoing publications do not disclose reducing drying time in a high black dot density area.
Further, according to the technique disclosed in the foregoing publication No. 5-338136, printing is made, taking into consideration black dot density and ambient temperature. However, this technique merely adjusts the drying time based on black dot density and ambient temperature, and the image forming rate may slow down depending on the image to be formed or ambient temperature. That is, this technique is not intended to actively reduce the drying time.
Therefore, this technique is bound to the problem of print contamination and see-through of black due to insufficient drying when the solid black area is formed using the slow-drying black ink to improve image quality while, at the same time, maintaining the image forming rate. Further, when these problems are to be solved by the foregoing technique, the drying time becomes longer under low-temperature conditions where ink viscosity is increased, and as a result recording speed becomes slow.
As described, the foregoing techniques of the prior art have various problems which are associated with drying of a solid black area in image formation.
On the other hand, the technique which provides the drying means has the problem of complex device structure and increased power consumption due to power consumed by the drying means.
Further, in ink-jet printers, generally, images are formed successively with respect to recording sheets which are continuously fed, and the recording sheets with images are successively discharged to a discharge tray and stacked thereon. In this case, in the event where subsequent recording sheets are discharged while the ink on the preceding recording sheet which was discharged previously has not been dried completely, there will be contamination of images due to contact between the recording sheets. In view of this problem, various proposals have been made to improve image forming operation, so that subsequent recording sheets are stacked after the ink on the preceding recording sheet is completely dried.
For example, the foregoing publication No. 5-338136 discloses a technique of calculating a black pixel ratio in an image to be formed and finding an ambient temperature of the device, and changing the transport speed of the recording sheet, which has been applied with an ink, based on the calculated black pixel ratio and the detected ambient temperature of the device, so as to ensure that the ink is dried on the recording sheet on the discharge tray before subsequent recording sheets are discharged.
Further, Japanese Unexamined Patent Publication No. 9-76591 (publication date: Mar. 25, 1997) discloses a technique of measuring the time required to dry the ink on a recording sheet which was discharged previously, and the elapsed time from the end of discharge of this recording sheet, so as to carry out intermittent transport operation of subsequent recording sheets in such a manner that the elapsed time exceeds the time required to dry the ink.
Further, Japanese Unexamined Patent Publication No. 5664/1999 (Tokukaihei 11-5664) (publication date: Jan. 12, 1999) discloses a technical idea wherein a discharge stacker is adapted to have a discharge support of plural stages, and recording sheets having been formed with images are replaced one after another in the stages of the discharge support, so as to delay the time of contact such that the recording sheets come into contact with each other after the ink has been dried.
However, in the technique disclosed in the foregoing publication No. 5-338136, while it takes into consideration black pixel ratio and ambient temperature of the device, it merely adjusts the drying time based on these variables. Thus, there were cases where the image forming rate slowed down depending on the image to be formed or ambient temperature of the device. Particularly, when the ambient temperature of the device is low, the image forming rate is decreased greatly. That is, this technique is not intended to actively reduce drying time of the ink.
Similarly, the technique disclosed in the foregoing publication No. 9-76591 is also for increasing the time required to form an image on a subsequent recording sheet, and there were cases where the image forming rate was decreased greatly depending on the image to be formed. That is, this technique is not for actively reducing drying time of the ink either.
Further, in the technique disclosed in the foregoing publication No. 11-5664, not only the structure of the discharge stacker is made complex but it requires a driving power to replace the recording sheets one after another in plural stages of the discharge support, and as a result power consumption of the entire image forming device may be increased.
As described, none of the foregoing prior art realizes stacking subsequent recording sheets after the ink on the previously discharged recording sheet is completely dried, without increasing the time required to form an image and without resulting in a complex discharge structure of the device.
Further, even though there have been proposals as above to provide drying means such as a heater to facilitate drying of the ink, this is not practical since it results in complex device structure and large power consumption by the drying means.
It is an object of the present invention to provide an ink-jet image forming method and an ink-jet image forming device which can omit or reduce the size of drying means which consumes a large amount of power and causes large increase in cost of the device, and which is capable of efficiently drying even a high dot density area while suppressing deterioration of image quality, and in particular to provide an ink-jet image forming method and an ink-jet image forming device which can create an image in a shorter period of time, and, at the same time, prevent contamination of recording sheets due to undried ink, without requiring drying means.
In order to achieve the foregoing object, an ink-jet image forming method in accordance with the present invention is adapted to form an image by forming dots using a slow-drying ink and a fast-drying ink having relatively longer drying time and shorter drying time, respectively, wherein an ink to be used to form a dot is selected from the slow-drying ink and the fast-drying ink by detecting and based on the ambient temperature of an area where the image is formed.
The ink to be used to form an image generally includes the slow-drying ink which has desirable reproducibility of color (e.g., black) but longer drying time, and the fast-drying ink which has inferior reproducibility of color but faster drying time.
The slow-drying ink has such properties that its viscosity changes depending on an ambient temperature of an area where an image is formed, and its permeation rate with respect to a recording sheet also changes depending on the ambient temperature. For example, the higher the ambient temperature, the faster the permeation rate, and the lower the ambient temperature, the slower the permeation rate.
Further, the permeation rate of the slow-drying ink has an influence on the drying time of the slow-drying ink, such that the faster the permeation rate, the shorter the drying time, and the slower the permeation rate, the longer the drying time.
Thus, the foregoing method selects an ink to be used to form dots from the slow-drying ink and the fast-drying ink based on an ambient temperature of an area where the image is formed. This allows for adjustment of an ink in accordance with temperature conditions of image formation, so as to use less slow-drying ink and use the fast-drying ink instead, thereby controlling drying time of the ink so that the ink is dried within a predetermined period of time. As a result, a print speed can be increased.
Further, the foregoing method may be adapted to adjust the use of an ink so as to use the slow-drying ink as much as possible within a range which allows the ink to dry within a predetermined period of time, thereby preventing deterioration of image quality by improving reproducibility of color.
As a result, it is possible to provide the ink-jet image forming method capable of preventing deterioration of image quality while increasing print speed.
It is preferable in the ink-jet image forming method of the present invention, in addition to the foregoing ink-jet image forming method, that dot density of an area which is defined in advance on the image with respect to the dot is recognized based on image data which is used to form the image, and the ink to be used to form the dot is selected based also on the dot density recognized.
The drying time of the dots formed with the slow-drying ink is also influenced by dot density of surrounding dots. That is, the higher the dot density, the longer the drying time, and the lower the dot density, the shorter the drying time.
Thus, the foregoing method is adapted to select an ink to be used to form dots, from the slow-drying ink and the fast-drying ink, based on dot density of dots which are formed in an area which is defined in advance with respect to dots to be formed in an image area, in addition to the ambient temperature of an area where the image is formed.
With this method, in an event where it is difficult to dry the ink, i.e., when the ambient temperature is low and the dot density is high, the dot density of the slow-drying ink can be lowered, for example, by partially using the fast-drying ink for the dots which are to be formed with the slow-drying ink, taking into consideration ambient temperature of image formation and dot density of the image to be formed. As a result, drying time of the ink can be reduced and the print speed can be increased.
Further, the dot density of the fast-drying ink may be increased as much as possible within a range which can maintain a required print speed, thus preventing deterioration of image quality.
As a result, it is possible to provide the ink-jet image forming method which can increase print speed and prevent deterioration of image quality further effectively.
In order to achieve the foregoing object, an ink-jet image forming device of the present invention, which is adapted to form an image by ejecting inks, includes a slow-drying ink head for ejecting the slow-drying ink having relatively longer drying time; a fast-drying ink head for ejecting the fast-drying ink having relatively shorter drying time; a temperature detecting device for detecting ambient temperature of an area where an image is formed; and a control device for selecting an ink head which ejects an ink, from the slow-drying ink head and the fast-drying ink head, based on the detected ambient temperature.
With this arrangement, an image can be formed by driving one of or both of the slow-drying ink head and the fast-drying-ink head by the control device based on the ambient temperature of an area where the image is formed, which was detected by the temperature detecting means. Thus, it is possible with this arrangement, as with the foregoing ink-jet image forming method, to prevent deterioration of image quality while increasing print speed.
The ink-jet image forming device of the present invention preferably includes, in addition to the foregoing arrangement of the ink-jet image forming device, a calculating device which calculates density of an ink to be ejected on a predetermined area of the image based on image data used to form the image, and the control device selects an ink head which ejects an ink, from the slow-drying ink head and the fast-drying ink head, based on the calculated density of the ink.
With this arrangement, the control device can select and drive the ink head based on ink density calculated by the calculating device. Thus, with this arrangement, as above, the print speed can be increased and deterioration of image quality can be prevented further effectively.
Further, in order to achieve the foregoing object, the present invention provides an ink-jet image forming method which is adapted to use a fast-drying ink together with a slow-drying ink, and which successively forms images with the fast-drying ink and the slow-drying ink on a plurality of recording sheets which are successively fed, and discharges the recording sheets so that a subsequent recording sheet is stacked on a preceding recording sheet, wherein: a drying time which is required to dry an ink applied to each of a plurality of image forming areas on the preceding recording sheet is controlled by adjusting, with respect to each image forming area, a ratio of the fast-drying ink to the slow-drying ink which are used to form an image, so that a rest time of the ink, which is a time period from an application of the ink to a time the subsequent recording sheet is stacked, is equal to or greater than the drying time with respect to each image forming area of the preceding recording sheet.
With this method, the subsequent recording sheet is discharged after the required drying time of the ink on the preceding recording sheet has elapsed. Therefore, it is possible to prevent contamination due to contact between recording sheets after image formation, without using additional means to dry the ink.
The ink-jet image forming device of the present invention includes a rest time recognizing section and an ink ratio adjusting section, which employ the foregoing image forming method. The rest time recognizing section recognizes the rest time of an ink, which is a time period from an application of the ink to the time the subsequent recording sheet is stacked, with respect to each of a plurality of image forming areas of the preceding recording sheet. The ink ratio adjusting section controls, upon receiving an output of the rest time recognizing section, a drying time which is required to dry the ink applied to each of the plurality of image forming areas on the preceding recording sheet, by adjusting, with respect to each image forming area, a ratio of the fast-drying ink to the slow-drying ink which are used to form an image, so that the rest time of the ink is equal to or greater than the drying time with respect to each image forming area of the preceding recording sheet.
With this arrangement, the ink ratio adjusting section adjusts, with respect to each image forming area, a ratio of the fast-drying ink to the slow-drying ink which are used to form an image, so that the rest time of the ink is not less than the rest time which is required for the applied ink to dry on each image forming area. As a result, the subsequent recording sheet is discharged after the ink on the preceding recording sheet is completely dried, thereby preventing contamination of recording sheets due to contact with one another in forming images by the present device.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.